Solution structure and reactivity of hydridoiron tetracarbonyl anion, [HFe(CO)4]-

Scaffold-based [Fe]-hydrogenase model: H2 activation initiates Fe(0)-hydride extrusion and non-biomimetic hydride transfer

We report the synthesis and reactivity of a model of [Fe]-hydrogenase derived from an anthracene-based scaffold that includes the endogenous, organometallic acyl(methylene) donor. In comparison to other non-scaffolded acyl-containing complexes, the complex described herein retains molecularly well-defined chemistry upon addition of multiple equivalents of exogenous base. Clean deprotonation of the acyl(methylene) C–H bond with a phenolate base results in the formation of a dimeric motif that contains a new Fe–C(methine) bond resulting from coordination of the deprotonated methylene unit to an adjacent iron center. This effective second carbanion in the ligand framework was demonstrated to drive heterolytic H₂ activation across the Fe(ii) center. However, this process results in reductive elimination and liberation of the ligand to extrude a lower-valent Fe–carbonyl complex. Through a series of isotopic labelling experiments, structural characterization (XRD, XAS), and spectroscopic characterization (IR, NMR, EXAFS), a mechanistic pathway is presented for H₂/hydride-induced loss of the organometallic acyl unit (i.e. pyCH₂–C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> O → pyCH₃+C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> O). The known reduced hydride species [HFe(CO)₄]⁻ and [HFe₃(CO)₁₁]⁻ have been observed as products by ¹H/²H NMR and IR spectroscopies, as well as independent syntheses of PNP[HFe(CO)₄]. The former species (i.e. [HFe(CO)₄]⁻) is deduced to be the actual hydride transfer agent in the hydride transfer reaction (nominally catalyzed by the title compound) to a biomimetic substrate ([^TolIm](BAr^F) = fluorinated imidazolium as hydride acceptor). This work provides mechanistic insight into the reasons for lack of functional biomimetic behavior (hydride transfer) in acyl(methylene)pyridine based mimics of [Fe]-hydrogenase.

We report the synthesis and reactivity of a model of [Fe]-hydrogenase derived from an anthracene-based scaffold that includes the endogenous, organometallic acyl(methylene) donor.

Low-Valent, Multiply Bonded, Trigonal-Planar Sb Complex: Rational Syntheses, Dual Acidic/Basic Properties, and Unexpected Semiconducting Characteristics

A 4-center, 6π-conjugated, multiply bonded trigonal-planar complex, [Sb{Cr(CO)₅}₃]^- (1), was synthesized via the hydride abstraction of [HSb{Cr(CO)₅}₃]^2- (1-H) with HBF₄·H₂O, with the release of high yields of H₂. The oxidation state of the Sb atom in [Et₄N][1] was well-defined as 0, which was evidenced by X-ray photoelectron spectroscopy and X-ray absorption near-edge structure. The distinct color-structure relationship of this low-valent Sb complex 1 toward a wide range of organic solvents was demonstrated, as interpreted by time-dependent density functional theory calculations, allowing the trigonal-planar 1 and the tetrahedral solvent adducts to be probed, revealing the dual acid/base properties of the Sb center. In addition, 1 showed pronounced electrophilicity toward anionic and neutral nucleophiles, even with solvent molecules, to produce tetrahedral complexes [(Nu)Sb{Cr(CO)₅}₃]^n- [1-Nu; n = 2, Nu = H, F, Cl, Br, I, OH; n = 1, Nu = PEt₃, PPh₃, N,N-dimethylformamide (DMF), acetonitrile (MeCN)]. On the contrary, the Fe/Cr hydride complex [HSb{Fe(CO)₄}{Cr(CO)₅}₂]^2- (2-H) was obtained by treating 1 with [HFe(CO)₄]^-. Upon hydride abstraction of 2-H with HBF₄·H₂O or [CPh₃][BF₄], a multiply bonded Fe/Cr trigonal-planar complex, [Sb{Fe(CO)₄}{Cr(CO)₅}₂]^- (2), was produced in which the oxidation coupling Sb₂-containing complexes [Sb₂Cr₄Fe₂(CO)₂₈]^2- (3-Cr) and [HSb₂Cr₃Fe₂(CO)₂₃]^- (3-H) were yielded as final products. Complex 3-Cr exhibited dual Lewis acid/base properties via hydridation and protonation reactions, to form 2-H or 3-H, respectively. Surprisingly, [Et₄N][1] possessed a low energy gap of 1.13 eV with an electrical conductivity in the range of (1.10-2.77) × 10^-6 S·cm^-1, showing that [Et₄N][1] was a low-energy-gap semiconductor. The crystal packing, crystal indexing, and density of states results of [Et₄N][1] further confirmed the efficient through-space conduction pathway via the intermolecular Sb···O(carbonyl) and O(carbonyl)···O(carbonyl) interactions of the 1D anionic zigzag chain of 1.

Synthesis and protonation of an encumbered iron tetraisocyanide dianion

Reported here are synthetic studies probing highly reduced iron centers in an encumbering tetraisocyano ligand environment. Treatment of FeCl2 with sodium amalgam in the presence of 2 equiv of the m-terphenyl isocyanide CNAr(Mes2) (Ar(Mes2) = 2,6-(2,4,6-Me3C6H2)2C6H3) produces the disodium tetraisocyanoferrate Na2[Fe(CNAr(Mes2))4]. Structural characterization of Na2[Fe(CNAr(Mes2))4] revealed a tight ion pair, with the Fe center adopting a tetrahedral coordination geometry consistent with a d(10) metal center. Attempts to disrupt the cation-anion contacts in Na2[Fe(CNAr(Mes2))4] with cation-sequestration reagents lead to decomposition, except for the case of 18-crown-6, where a mononuclear complex featuring a dianionic 1-azabenz[b]azulene ligand was isolated in low yield. Formation of this 1-azabenz[b]azulene is rationalized to proceed by an aza-Büchner ring expansion of a CNAr(Mes2) ligand mediated by a coordinatively unsaturated Fe center. Disodium tetraisocyanoferrate Na2[Fe(CNAr(Mes2))4] is readily protonated by trimethylsilanol (HOSiMe3) to produce the monohydride ferrate salt, Na[HFe(CNAr(Mes2))4], the anionic portion of which serves as an isocyano analogue of the hydrido-tetracarbonyl metalate [HFe(CO)4](-). Treatment of Na[HFe(CNAr(Mes2))4] with methyl triflate (MeOTf; OTf = [O3SCF3](-)) at low temperature in the presence of dinitrogen yields the five-coordinate Fe(0) complex Fe(N2)(CNAr(Mes2))4. The formation of Fe(N2)(CNAr(Mes2))4 in this reaction is consistent with the intermediacy of the neutral tetraisocyanide Fe(CNAr(Mes2))4. The decomposition of Fe(N2)(CNAr(Mes2))4 to the dimeric complex [Fe(η(6)-(Mes)-μ(2)-C-CNAr(Mes))]2 and a seven-membered cyclic imine derived from a CNAr(Mes2) ligand is presented and provides insight into the ability of CNAr(Mes2) and related m-terphenyl isocyanides to stabilize zerovalent four-coordinate iron complexes in a strongly π-acidic ligand field.

An anionic phosphenium complex as an ambident nucleophile

Treatment of an N-heterocyclic chlorophosphine with Collman's reagent or K[HFe(CO)₄]/NaH gave a unique anionic phosphenium complex which behaves as an ambident nucleophile.

Direct Lanthanide-Transition Metal Interactions: Synthesis of (NH(3))(2)YbFe(CO)(4) and Crystal Structures of {[(CH(3)CN)(3)YbFe(CO)(4)](2).CH(3)CN}(infinity) and [(CH(3)CN)(3)YbFe(CO)(4)](infinity)

The heterometallic complex (NH(3))(2)YbFe(CO)(4) was prepared from the reduction of Fe(3)(CO)(12) by Yb in liquid ammonia. Ammonia was displaced from (NH(3))(2)YbFe(CO)(4) by acetonitrile in acetonitrile solution, and the crystalline compounds {[(CH(3)CN)(3)YbFe(CO)(4))](2).CH(3)CN}(infinity) and [(CH(3)CN)(3)YbFe(CO)(4)](infinity) were obtained. An earlier X-ray study of {[(CH(3)CN)(3)YbFe(CO)(4)](2).CH(3)CN}(infinity) showed that it is a ladder polymer with direct Yb-Fe bonds. In the present study, an X-ray crystal structure analysis also showed that [(CH(3)CN)(3)YbFe(CO)(4)](infinity) is a sheetlike array with direct Yb-Fe bonds. Crystal data for {[(CH(3)CN)(3)YbFe(CO)(4)](2).CH(3)CN}(infinity): monoclinic space group P2(1)/c, a = 21.515(8) Å, b = 7.838(2) Å, c = 19.866(6) Å, beta = 105.47(2) degrees, Z = 4. Crystal data for [(CH(3)CN)(3)YbFe(CO)(4)](infinity): monoclinic space group P2(1)/n, a = 8.364(3) Å, b = 9.605(5) Å, c = 17.240(6) Å, beta = 92.22(3) degrees, Z = 4. Electrical conductivity measurements in acetonitrile show that these acetonitrile complexes are partially dissociated into ionic species. IR and NMR spectra of the solutions reveal the presence of [HFe(CO)(4)](-). However, upon recrystallization, the acetonitrile complexes show no evidence for the presence of [HFe(CO)(4)](-) on the basis of their IR spectra. The solid state MAS (2)H NMR spectra of deuterated acetonitrile complexes give no evidence for [(2)HFe(CO)(4)](-). It appears that rupture of the Yb-Fe bond could occur in solution to generate the ion pair [L(n)Yb](2+)[Fe(CO)(4)](2-), but then the highly basic [Fe(CO)(4)](2-) anion could abstract a proton from a coordinated acetonitrile ligand to form [HFe(CO)(4)](-). However, upon crystallization, the proton could be transferred back to the ligand, which results in the neutral polymeric species.